Atmospheric pollution is a societal problem which is receiving much attention. The major source of such pollution is the extensive use of fossil fuels, although industrial and chemical processes, such as the manufacture of nitric acid, also contribute. The principal pollutants are nitrogen oxides, carbon monoxide and, perhaps to a lesser extent, hydrocarbons, sulfur oxides and other objectionable gases and vapors.
Although several nitrogen oxides are known which are relatively stable at ambient conditions, it is generally recognized that two of these, nitric oxide (NO) and nitrogen dioxide (NO.sub.2), are the principal contributors to smog and other undesirable environmental effects when they are discharged into the atmosphere.
Formation of man-made nitrogen oxides from the elements occurs in the high temperature zones of combustion processes. The internal combustion engine and coal-, oil- and gas-fired furnaces, boilers and incinerators all contribute to NO.sub.x emissions. Although the concentrations of NO.sub.x in the exhaust gases produced by combustion usually are low, the aggregate amounts discharged in industrial and/or highly populated areas is adequate to cause problems. For purposes of the present invention, the nomenclature NO.sub.x will be used herein to represent nitric oxide, nitrogen dioxide, other nitrogen oxides and mixtures thereof.
The reduction of obnoxious nitrogen oxides to environmentally benign nitrogen in the emissions from lean burn engines cannot be achieved with the type of catalysts that are now used in most passenger cars in the United States, because such catalysts do not function under oxidizing conditions; whereas, lean burn emissions always contain substantial amounts of free oxygen. It is widely assumed that cars of the future should use lean burn engines because of their superior fuel economy. For treating the emissions from such engines, a different class of catalysts has been considered; they contain copper or other metal ions in zeolite cavities. They are active under oxidizing conditions; however, their activity is drastically suppressed by water vapor which is, of course, an inevitable ingredient of all internal combustion engine emission gases. Recently, a type of catalyst has been prepared by a narrowly specified ion exchange procedure in an aqueous slurry of iron oxalate and the zeolite ZSM-5. These catalysts are claimed to operate in gases that contain both free oxygen and a substantial content of water vapor. However, they appear to require a higher operating temperature than is desirable or necessary for treating emissions from lean burn or diesel engines.